The present invention relates to exhausting air from a heating, ventilating and air conditioning system; and in particular to the exhaust control of an air handling unit.
Heating, ventilating and air conditioning (HVAC) systems are well known for maintaining environmental conditions within buildings. A typical HVAC installation divides the building into zones and regulates environmental parameters (e.g. temperature, humidity, outdoor/recirculated air ratio, etc.) of each zone to predefined control setpoints. An air distribution system connects each of the zones to a separate air handling unit (A.U.) that conditions the air supplied to a particular zone. The air handling unit generally includes elements for introducing outdoor air into the system and for exhausting air from the system. Other elements are provided for heating, cooling, filtering and otherwise conditioning the air for the zone.
The air handling unit has a constant volume supply fan to circulate the air within the zone's air distribution ducts at a desired flow rate. A pair of electrically operated dampers control the amounts of outdoor air and recirculated air that are fed into the zone. Typically these dampers are operated by a common actuator so that as one damper closes the other damper opens, and vice versa. As a consequence of introducing outdoor air into the zone, a varying volume of air is added to the zone. That same volume of air has to be exhausted or the pressure within the zone will increase to unacceptable levels.
Thus a barometric relief damper typically is built into the air handling unit to allow air to flow from the system to the outdoors. The relief damper can be implemented by a series of vertically mounted, hinged metal blades that remain closed due to gravity when air is not flowing through the system. Air flow through the system causes the blades open when a significant difference between the static air pressure in the zone and the outdoor atmospheric pressure exists. The actual pressure differential across the damper may be less than that pressure difference due to flow resistance in the return air ducts. The amount of air relieved by a barometric relief damper is a function of the damper's flow-through area A and the pressure differential .DELTA.P as given by the expression: EQU FLOW=k.sub.1 A.sqroot.k.sub.2 .DELTA.P (1)
where k.sub.1, and k.sub.2 are constants. Although a slight positive pressure (e.g. 0.05" w.g.) within the zone is beneficial, too large a pressure differential results in difficulty operating doors and whistling noises due to air escaping the zone.
Outdoor air is drawn into the air handling unit by an internal fan and thus the maximum amount of air that has to be exhausted through the barometric relief damper is equal to the fan's airflow when the outdoor air damper is fully opened. However, the typical barometric relief damper cannot relieve that much air flow unless the pressure differential reaches an unexceptionably high level, in some instances more than ten times an acceptable pressure level within the zone. Thus, in typical air handling units it is impractical to build a barometric relief damper with a flow-through area that is large enough to relieve the maximum air flow without exceeding a 0.05" w.g. static pressure in the zone.
As a solution, some air handling units provide a power exhaust fan on the outside of the barometric relief damper. When the power exhaust fan operates, air is drawn through the barometric relief damper due to an increased pressure differential across that damper, thereby increasing the relief air flow without an increase in the zone static pressure. The resulting air flow depends on the power exhaust fan's flow characteristic, as well as on the barometric relief damper flow characteristic. The typical power exhaust fan is used to boost the relief air flow only at times where the outdoor air inlet damper was opened fully, that is at times when the amount of the outdoor air brought into the space is fairly large. If the relief air flow from the power exhaust fan is too great, the zone static air pressure decreases to a negative level (less than atmospheric pressure) with the undesirable effect that outside air infiltrates the zone. Such infiltration may occur through other building envelope openings.
Current industry practice is to size the exhaust fan to be capable of relieving the air handling unit's maximum outdoor air inflow, as occurs when the outdoor air supply damper is fully open. Thus, the power exhaust fan turns on only when the outdoor air damper is open all the way. Typically, a switch for controlling the exhaust fan is mounted on the outdoor air supply damper and closes when that damper reaches the fully open position. However, this control does not permit the exhaust fan to relieve air when the outdoor air supply damper is less than fully opened. As noted previously, if that exhaust fan is activated at a lower flow of outdoor supply air a negative pressure will be created within the zone.
Some air handling units attempted to cure this problem by utilizing relatively expensive, variable speed/variable flow capacity power exhaust fan systems. Such systems require a variable speed motor controller with a pressure sensor located within the zone to provide closed loop static pressure control of the fan. The power exhaust fan still is sized to exhaust 100% of the outdoor air that is added by the supply damper being fully opened, but the speed of the fan is controlled to maintain an acceptable static pressure level within the zone. However, it is desirable to provide a less expensive and less sophisticated control of the exhaust air flow.